Geostationary earth orbit (GEO) is 22,282 miles above the equator. The orbit is important because it allows a satellite to orbit the earth at a fixed location in relation to the earth. From GEO, three satellites can cover all of the earth's surface excluding the polar regions, and transmissions can be received through fixed antennas. Once an antenna has been correctly aligned with the satellite, no further tracking issues arise since the satellite remains in the same relative position.
Transmissions via GEO are subject to a delay which is noticeable particularly in respect of voice communication. A relatively large amount of transmission power is needed due to the long distances involved and a large dish may be required for sending and receiving, especially to achieve high data rates.
Systems are also in place for placing satellites in lower orbits. The lower orbits may be categorized in two groups, low earth orbit (LEO) and medium earth orbit (MEO). LEO communication systems are based on a constellation of small low earth orbiting satellites orbiting in the range of 500 miles above the earth. The constellation is preferably sufficiently large to provide global coverage in that every position on the earth's surface is in site of one of the satellites at any given time. Systems, such as Iridium, ICO, Globalstar, Teledesic and Skybridge have been available providing two-way links via LEO, some of which can support Internet communication, but these have been notable for lack of commercial success. The only system currently operating commercially is Globalstar which is narrowband and currently used mainly for voice. ICO is being redesigned for packet communication with data rates of up to 144 kbs and Skybridge is designed for high data rates.
Skybridge and Globalstar both use transparent transponders and communication is controlled by a network of terrestrial gateways.
The advantages of LEO include no discernable time delay in communication and the absence of the need for heavy transmission power or a large receiver dish. More generally it provides a better trade off between antenna size, transmitter power and data rate than does a GEO based system.
The disadvantages of LEO based systems in general are that a steerable antenna is needed, at least for high data rates, and the available bandwidth is relatively narrow, particularly at L band frequencies and particularly with the ICO and Global Star systems.
MEO covers orbital heights in between GEO and LEO and provides a compromise between the advantages and disadvantages of each.
Increasingly, GEO-based satellite communications systems are being made to be compliant with the International DVB standard, principally intended for digital television broadcasting but now also embracing the Internet communication protocol TCP/IP. The DVB standard is based on the common MPEG-2 coding system and is designed to produce signals which are easily transferable from one medium to another, frequently needed in today's complex signal distribution environment. DVB signals move easily and inexpensively from satellite to cable, from cable to terrestrial radio and to the telephone network. A part of the DVB standard specifically for satellite forward link (to the user) is known as DVB-S and for satellite return link (from the user) is known as DVB-RCS.
Essentially, thanks to the use of MPEG-2 packets as the principle data carrier, and the way in which control information is arranged therein, DVB can deliver almost anything that can be digitized, including High Definition TV, multiple channel Standard definition TV (PAL/NTSC or SECAM), broadband multimedia data and interactive services.
GEO based systems comprise much equipment which is DVB-S compliant and thus it is desirable to use GEO for data communication. On the other hand two-way data communication requires a return channel, from the remote user back to the network. Often the remote channel is very lightly used. Most Internet users download the vast majority of their data from the network and upload relatively little. Use of GEO for the return channel requires a significant transmission capability which may not be regarded as justified for the amount of traffic involved.
Thus a system has been proposed in which GEO provides the link from the Internet to a remote user but a return link is provided via telephone. The solution however may require paying connection charges for two links, the telephone return channel may be slow and telephone lines are not universally available.